Field of the Invention
The present invention generally relates to methods for growing single crystal GaN (Gallium Nitride) on a semiconductor substrate and, more particularly, to methods for growing single crystal GaN on a silicon wafer.
GaN and its alloys are promising as a wide band-gap, high temperature semiconductor material suitable for optoelectronic applications such as blue- and ultraviolet-light-emitted devices, and high power/high frequency devices, such as piezoelectric resonators, RF transistors and lasers. Currently, single crystal GaN film can be grown on a sapphire (xcex1-Al2O3) substrate with a (0 0 0 1) orientation. However, the large lattice and thermal mismatch between GaN film and sapphire leads to high defect density in the grown GaN film that deteriorates the optoelectronic properties of the GaN film. In addition, sapphire is not conductive and is difficult to integrate with other semiconductor devices. Accordingly, after formation of GaN film, the GaN film needs to be removed from the sapphire substrate for further processing.
A recent development is to grow GaN layers on a SiC (silicon carbide) substrate. SiC possesses a wide-band-gap with high thermal stability, excellent resistance to chemical attack, high thermal conductivity, high electron mobility, and relatively small lattice mismatch with GaN. However, SiC is very expensive and is typically available only in smaller diameter wafers. Although SiC is conductive and is relatively xe2x80x9cmatchablexe2x80x9d with GaN, it is difficult to obtain a high quality, large size GaN film on a SiC substrate at a low cost. Moreover, recent GaN on SiC fabrication. techniques are not able to grow a 1 micron or greater unmasked GaN film without cracking. It is possible, however, to grow GaN on selected substrate areas with special patterns (xe2x80x9cislandsxe2x80x9d), to achieve a thickness of up to 2 microns of GaN. Unfortunately, cracks and pits still often develop on the grown GaN when the grown, GaN on SiC is cooled down to room temperature. Furthermore, this technique requires several masking or etching steps, which is complicated and time consuming.
Compared with the sapphire and SiC substrates mentioned above, a silicon (Si) substrate is the most inexpensive and most promising for growth of GaN layer. A Si substrate not only has the advantages of low cost and good electrical and thermal conductivity, but also is available in larger wafer size. Further, GaN epitaxy on Si facilitates integration of microelectronics and optoelectronics. However, it is difficult to grow single crystal GaN directly on a Si substrate because of large mismatches between GaN and Si. Besides the large lattice mismatches between GaN and Si, there is a more significant problem of a larger thermal expansion coefficient of GaN than that of Si that limits successful heteroepitaxy. Therefore, the crystal quality of GaN-on-Si is still inferior to that of GaN layers grown on sapphire or SiC substrates.
Several techniques have been developed to solve the above problems in growing GaN on Si. For example, it has been suggested that a GaN epitaxy be grown on Si(1 1 1) by a vacuum reactive evaporation method or a MetalOrganic Chemical Vapor Deposition (MOCVD) using buffer layers of SiC and AlN, respectively. It has also been suggested that a cubic GaN layer be grown on a Si(0 0 1) substrate by plasma-assisted molecular beam epitaxy (MBE), or wurtzite GaN be grown on a Si3N4 buffer layer formed on a Si(1 1 1) substrate. However, none of these suggested techniques can grow a GaN film with a thickness of over 2 microns without cracks or pits over an entire surface of the Si substrate.
Accordingly, one object of the present invention is lo provide a method for growing a single crystal GaN film on a Si(1 1 1) substrate. The single crystal GaN is grown by a MOCVD method and includes a sequence of AlN and GaN layers alternatively deposited on the Si substrate. The deposition conditions and timings of these layers are controlled so that the single crystal GaN film can be formed thicker than 2 microns on the Si(1 1 1) substrate without cracks or pits.
In accordance with one embodiment of the invention, a method for growing a single crystal GaN (gallium nitride) film on a Si substrate comprises depositing a buffer layer on a top of the Si substrate using Trimethylaluminum (TMAl) in a hydrogen atmosphere, and depositing alternately a number of GaN layers and interlayers on top of the buffer layer.
In accordance with another embodiment of the present invention, each of the interlayers includes at least three layers: an AlN layer, a GaN layer and an AlN layer, from bottom to top. The AlN layer and the GaN layer in each interlayer are formed using TMAl and NH3 in a hydrogen atmosphere and TMGa and NH3 in a hydrogen atmosphere, respectively. Each of the AlN layer and the GaN layers within each interlayer has a thickness of about 100 xc3x85.
In accordance with yet another embodiment of the present invention, the buffer layer, the GaN layers and the interlayers are formed by MOCVD at a pressure of about 100 Torr. The thickness of the buffer layer is about 400 xc3x85 and the thickness of each GaN layer is about 5000 xc3x85.
In accordance with still another embodiment of the present invention, a single crystal GaN film structure comprises a Si(1 1 1) substrate, a buffer layer deposited on the Si(1 1 1) substrate that is formed by a metalorganic chemical vapor deposition (MOCVD) by using a trimethylaluminum (TMAl) as a reactive gases, and a plurality of GaN layers and interlayers alternatively deposited on the buffer layer with a GaN layer directly deposited on a top of the buffer layer and each of the interlayers interspaced between two GaN layers.
In accordance with still another embodiment of the invention, a Si substrate is prepared by first bathing a Si wafer in BOE (buffered oxide etch) etchant (10:1) for up to 1 minute. The wafer is then rinsed with deionized (DI) water for up to 10 minutes. The wafer is then removed from the DI water and any residual water is removed with a nitrogen gas steam. Afterward, the Si wafer is loaded into a MOCVD reactor. A prelayer is formed on the Si wafer after the baking step. The prelayer is deposited at a pressure of about 100 Torr in a hydrogen atmosphere and at a temperature of about 1210 degrees C. for about 4-8 seconds.
The features and attendant advantages of the present invention will be more fully appreciated upon a reading of the following detailed description in conjunction with the accompanying drawings.